I have my gmail account set up to pull all my mail automatically from my Yahoo email account. The problem is, if somebody sends an e-mail to my yahoo account, and I read the message on my blackberry, my gmail account doesn't 'mark as read' the message. When I log directly into my Yahoo account, Yahoo knows that I've read the message.
Nothing, it's just the way it is. My Gmail does the same thing. It's kind of annoying but it's not a big enough problem to worry about. That's my thought any way.